1. Field of the Invention
The present invention relates to additives, cementing compositions and methods of use and, more particularly, but not by way of limitation, to additives, cementing compositions and methods for use in oil, gas, and geothermal wells.
2. Description of the Related Art
In downhole cementing operations, such as those that occur in oil, gas, and geothermal wells, it is known to use cementing compositions that contain, among other ingredients, a hydraulic cement and a latex (or lattice). A latex is a dispersion of organic polymer particles in water. Most latexes are milky white to off-white in color and vary in consistency or viscosity from low viscosity, water-thin fluids to very viscous liquids. The basic components of a latex are a polymer composition, surfactants and, in many cases, compounding ingredients.
The polymer composition, molecular weight, and particle sizes of the organic polymer in the dispersion have a significant effect on the properties of the liquid cement slurry as well as the hardened or cured cement. Because most latexes are made through a process of emulsion polymerization, with the exception of epoxy resin latexes, surfactants are present and a key ingredient in the latex. Most latexes contain a nonionic surfactant and an anionic surfactant. The nonionic surfactant is typically, but not limited to, a nonylphenol with 10 to 40 moles of ethoxylation and is the primary emulsifier. The concentration of nonionic surfactant typically ranges from 1 to 10 percent by total weight of the latex dispersion. Anionic surfactants are used at much lower concentrations, typically between 0.1 and 2 percent. Anionic surfactants function to control the rate of polymerization of the monomers being reacted to form the latex polymer.
Compounding ingredients are added after polymerization is complete to improve the latex product for the application. Compounding ingredients include bactericides, defoamers, antioxidizing agents, ultraviolet light (UV) stabilizers, and additional surfactants to improve workability of a cement formulation, improve freeze-thaw stability, reduce water-to-cement ratio, etc. Many surfactants added to improve workability (viscosity or consistency of the liquid cement slurry) or reduce the water-to-cement ratio function as dispersants for the cement particles.
The majority of latex types that have been or are being used with hydraulic cements, such as Portland cement, are: polyvinyl acetate, acrylic copolymers, styrene acrylic copolymers, vinyl acetate acrylic copolymers, vinyl acetate ethylene copolymers, vinylidene chloride and vinyl chloride copolymers, styrene butadiene copolymers (SB), and epoxy resin latexes. Each type of latex imparts different properties when used as an additive or polymeric modifier to hydraulic cement mixtures.
One of the most common latexes used in oil, gas and geothermal cement formulations is styrene butadiene (SB) latex. The most widely practiced application of styrene butadiene latexes is for prevention or control of gas migration or channeling after cementing based upon the art described in U.S. Pat. Nos. 4,537,918, 4,721,160 and 4,767,460 by Parcevaux et al. The art described by Parcevaux et al. in these patents is essentially a combination of the art described in U.S. Pat. Nos. 3,228,907, 4,151,150, and 4,039,345. Gas migration occurs when the well traverses a pocket of compressed gas and after a cement slurry has been injected into the well (either into the annular space between the casing and the borehole wall or interiorly of the casing). Gas migration or channeling occurs during the setting of the cement; from the time when setting of the cement has progressed such that the hydrostatic pressure of the cement column is no longer transmitted to the pocket of compressed gas but prior to the slurry sufficiently setting to oppose the migration of the gas into the setting cement under the pressure from the compressed gas pocket. The migrating gas permeates the cement during the course of its setting, creating a multiplicity of channels that may reach up to the surface of the well. Gas channeling can be a serious drawback, leading to weakening of the cement and to safety problems on the surface. In addition to preventing gas channeling or migration, SB latexes serve to increase adhesion of the cement to the casing and the formation, reduce fluid loss, and increase the elasticity and flexural strength of the set cement.
The key learning from the related art regarding the application of styrene butadiene latexes can be described as follows. First, the latexes are copolymers of styrene and butadiene having a styrene to butadiene weight ratio of about 30:70 to 70:30. This range is preferred because of the mechanism of action of latex for improved bonding and control of gas migration requires a latex that effectively forms films around the cement particles and coalesces when contacted by gas. Copolymer latexes with styrene content greater than about 70 percent do not form films that will provide the required mechanism of action. Copolymer latexes with a butadiene content greater than about 70 percent are so inherently unstable that, although they form effective films, they are for all practical purposes impossible to stabilize (control coagulation of the latex) in the presence of divalent ions present in cement slurries and at elevated temperatures. Essentially, SB latexes in cement compositions aimed at curtailing gas migration or channeling are generally limited to use at low temperatures (e.g., less than 200.degree. F.) or require stabilizers. Furthermore, without stabilizers, particularly at high pH levels, SB latexes tend to flocculate.
Second, the mechanism of improved cement bonding is through the interaction of the latex coating of the cement particles with (a) the geologic formation of the borehole wall or drilling fluid filter cake deposited on the borehole wall and (b) with the surface of the steel, fiberglass or other material of construction for the well casing. Styrene butadiene copolymer latexes provide a natural adhesion to solids because of their film forming tendencies. Further, the coating of the particles and films formed between cement grains and casing or borehole wall surfaces effectively increase the contact surface area of the cement slurry. Since shear bond strength of cement is a direct function of surface area, effectively increasing the surface area directly increases the shear bond strength between the cement and surrounding surfaces.
Third, styrene butadiene copolymer latexes are inherently unstable in cement slurries and particularly at the elevated temperatures typically associated with well cementing. Temperature, the shear of mixing and pumping the cement slurry, the concentration of electrolytes, such as chloride salts of alkali earth metals (sodium chloride, potassium chloride and calcium chloride by example) and formation fluids such as brines, carbon dioxide, hydrogen sulfide, natural gas and oil all affect the stability of the latex during and after placement of the cement slurry. The fundamental cause of this instability is the stability of the latex emulsion itself. The type and quantity of surfactants used in the manufacture of the latex are selected for the stability of the emulsion of the two monomers (styrene and butadiene) in the polymerization process to form the latex copolymer. Additional surfactants of same or different types used in the preparation of the latex are added to stabilize the emulsion for its intended use. This is well known to those practiced in the art of coatings and application of latex modified cement coatings for construction industry applications. An example of this is in U.S. Pat. No. 4,039,345. Parcevaux et al (U.S. Pat. No. 4,767,460) simply selected suitable surfactants compatible with the electrolytes present in cement slurries and which were effective to stabilize the copolymer emulsion at elevated temperatures.
Fourth, the fundamental instability of styrene butadiene copolymer latexes is necessary to provide film forming necessary to control gas migration through a cement slurry. When gas invades the cement, it naturally attempts to flow through the permeability of the slurry. Parcevaux et al (U.S. Pat. No. 4,537,918) described a `selective` film-forming response to gas by the latex to immediately inhibit further channeling or movement of the gas through the cement slurry.
Although not well understood or defined at the time by Parcevaux and others skilled in the art of well cementing, the selective film-forming response to gas by the styrene butadiene copolymer latex is a two-step process. Initiation of the process of gas invasion into the cement is the entry of small gas bubbles into the cement slurry. The gas bubbles must be small enough to enter the natural pores of the cement slurry. Migration of the small bubbles is limited because the force required to control movement of the bubbles, according to Stoke's Law, is small due to the small radius of the bubble. The gel strength of the slurry, the strength of the latex films between cement particles and/or the effective hydrostatic provided by the cement slurry or other fluid column above the cement in the annulus is sufficient to prevent movement of these small bubbles through the matrix porosity and permeability of the cement. As more small bubbles invade, some of the small bubbles combine to form larger bubbles. According to Stoke's Law, the force required to prevent migration of the gas bubble increases directly as a cubic function of the bubble radius. This process of combination of gas bubbles increases until the bubble is large enough to begin moving through the cement slurry, not just through the normal pore spaces within the cement. Large bubbles have sufficient force to overcome the gel strength or electrostatic attraction between cement particles in the slurry and thus force the cement grains apart to form a gas channel in the cement.
The selective film forming process described by Parcevaux is effective at all stages of gas invasion but is particularly more effective prior to the formation of a gas channel. At the time small bubbles begin to combine and move through the cement, the latex films between cement grains act as a barrier to movement of the bubbles, The self-adhesive properties of the latex film provide additional resistance to flow of the gas through the cement slurry above the gel strength force between cement grains. In fact, the latex film coating the cement particles tends to prevent the natural gel strength or electrostatic attraction between the surface of the cement grains. This accounts for the improved rheological properties (low gel strengths) observed by Parcevaux. As the gas bubble begins to move, the film of latex between the cement grains is expanded. If the force exerted by the bubble is sufficiently high, the latex film may be ruptured. Herein lies the first part of the selective film forming response of the latex as referred to by Parcevaux. When the stable latex film ruptures, it typically ruptures in multiple points. This effectively divides the large gas bubble into smaller bubbles that often exert a force below the rupture strength of the latex films. This process can be repeated until a large bubble has been subdivided into sufficiently small bubbles to become immobilized. The second part of Parcevaux's selective film forming response occurs during the process of the rupture of the latex film. As the gas bubble attempts to force its way through the latex film, the gas acts to dehydrate or dry the latex. The result is a coalescence and coagulation of some of the latex in the film. The coagulated latex precipitates from the film forming a viscous elastomeric mass that plugs part of the pore space of the cement slurry yet remains compatible with the uncoagulated latex. As a result, the coagulated latex becomes another solid across which new films may be formed by the remaining latex. This part of the selective film forming response is directly related to the concentrations of latex required to effectively control gas migration cited by Parcevaux. Sufficient latex must be present to allow some coagulation as well as maintain a volume of uncoagulated latex to re-form films between solids in the slurry. The action of dehydrating, coalescing and coagulating latexes is well know to those skilled in the art of latex paints, paper and textile coatings.
Fifth, styrene butadiene copolymer latexes reduce the fluid loss of cement slurries through a mechanism of film forming between cement grains. Stability of the films is critical to maintaining fluid loss control. Coagulation of the latex eliminates the films and unless sufficient additional latex is present which is uncoagulated, the fluid loss of the slurry increases dramatically.
Sixth, the improved rheological properties imparted by styrene butadiene copolymer latexes are also a function of the films coating the cement grains. This limits the development of gel strength or magnitude of electrostatic forces between cement grains. Additionally, the anionic surfactants present in the latex are also dispersants for cement particles and tend to neutralize some of the surface charges on the cement grains.
Finally, styrene butadiene copolymer latexes generally require a stabilizer for application in well cementing formulations to control or prevent coagulation of the latex. Stabilizers are especially required at elevated temperature (generally over 100 (F) and in the presence of salts, gas, oil and high surface area solids such as clays (bentonite attapulgite and sepiolite by example). Surfactants, sequestering agents and some low molecular weight resins can stabilize latexes. Low molecular weight resins that are stabilizers for styrene butadiene copolymer latexes cited in prior art include anionic polyelectrolytes such as sulfonated, sulfated, or sulfited melamine-formaldehyde, naphthalene-formaldehyde or phenol formaldehyde resins with molecular weights between about 200 and 10,000. Also, polyamido-sulfonic polymers having molecular weights in the same range.
Because cement dispersants and retarders generally fall within the chemical classification of surfactants, sequestering agents and low molecular weight resins optimum stabilization of a styrene butadiene copolymer latex is extremely complex. Latex stabilization is so complex that changes in the basic cement chemistry may impact stabilization properties of the latex. As those skilled in the art of well cementing know, cements vary significantly between manufacturer and between each mill run batch for a given manufacturer and cement plant. This is due to variations in raw materials, raw material blends, heating time and heat distribution within the cement kiln. Therefore, the stability of a latex can vary significantly even when other components of the formulation remain the same. Controlling the selective film forming response of the latex for well conditions is difficult and overstabilization of the latex produces stable films that do not effectively coalesce and coagulate when contacted by gas. This accounts for the lack of complete success of styrene butadiene copolymer latexes for controlling gas migration within the industry after nearly twenty years of use.
Accordingly, there is a need for cement compositions containing an SB latex that exhibit stability at high temperatures (i.e., above about 200.degree. F.), do not require stabilizers, and are effective at preventing gas migration or channeling.